A tapered roller bearing comprises inner and outer races having conical raceways, respectively, and tapered rollers disposed between the inner and outer races and received in pockets of a conical tubular retainer. Such a tapered roller bearing ordinarily includes a large flange formed on the inner race at the large-diameter end of its raceway for guiding the tapered rollers, and a small flange formed on the inner race at the small-diameter end of its raceway to restrict the axial position of the tapered rollers, thereby preventing separation of the tapered rollers from the inner race with the tapered rollers received in the respective pockets of the retainer during handling of the bearing.
Transmissions in today's vehicles have a larger number of steps for fuel economy, and their engine rooms are smaller to increase the passenger interior space. As a result, it is required to reduce the size of tapered roller bearings supporting power transmission shafts of vehicle differentials and transmissions because their available installation spaces are reduced. Simultaneously, to cope with increased output of vehicle engines, bearings with large load bearing capacity are required.
Some tapered roller bearings have a (large) flange only at the large-diameter end of the inner raceway to guide the tapered rollers while omitting the small flange. Instead, a radially inwardly extending protrusion is provided at the large-diameter end that axially engages the flange of the inner race, thereby indirectly restricting the axial position of the tapered rollers through the retainer so as to prevent separation of the tapered rollers from the inner race (see e.g. JP Utility Model Publication 58-165324 and JP Patent Publication 2002-54638A). In the invention disclosed in JP Utility Model Publication 58-165324, in order to easily form the inner race and to improve the yield of material, an engaging portion of the flange of the inner race for engaging a protrusion of a resin retainer is formed on a circumferential step provided on the corner between the outer end of the outer periphery of the flange and its end surface, on a circumferential groove formed in the outer periphery of the flange, or on a side surface of the flange. In the invention disclosed in JP Patent Publication 2002-54638A, in order to increase load bearing capacity of the bearing by increasing the width of the raceways and the length of the tapered rollers, an engaging portion of the flange of the inner race for engaging a protrusion of a retainer made of metal is formed on a circumferential step provided on the corner between the outer end of the outer periphery of the flange and its end surface.
In the arrangement in which, as in the tapered roller bearings disclosed in JP Utility Model Publication 58-165324 and JP Patent Publication 2002-54638A, instead of the small flange, a protrusion formed on the retainer is brought into engagement with the circumferential step or circumferential groove formed on the flange, and stress tends to concentrate on the corner of the circumferential step or groove. Thus, the strength of the retainer tends to be insufficient relative to thrust loads applied from the large end surfaces of the tapered rollers to the flange. In the arrangement in which the protrusion of the retainer is brought into engagement with the side surface of the flange, no stress concentration occurs. But since the protrusion of the retainer protrudes from the side surface of the flange, it is difficult to reduce the size of the bearing.
In the arrangement in which, as disclosed in JP Patent Publication 2002-54638A, the retainer having the protrusion is made of a metal, a bending step is necessary to form the protrusion, which pushes up the manufacturing cost. Also, the metallic retainer may produce metal abrasion dust which could deteriorate lubricating properties of the lubricating oil. Thus, resin retainers are becoming popular today.
When forming conventional retainers for tapered roller bearings from resin, resin is injected into a cavity defined by two axially separable molds 81 and 82 shown in FIG. 12. The two molds 81 and 82 are slid in the axial direction of the retainer 83 to be formed toward and away from each other.
In forming the tapered roller bearing having such a protrusion using the above-mentioned molds, in order to release the molds 81 and 82 after abutting them together as shown in FIG. 13, the inner diameter D of the radially inner end of the protrusion 83a at the large-diameter end of the retainer 83 has to be larger than the outer diameter D1 of the small-diameter annular portion 83b of the retainer 83. The outer diameter D1 of the small-diameter annular portion 83b is determined by the inclination angle of the conical tubular portion of the retainer, which is an intermediate value between the inclination angles of the raceways of the inner and outer races, and by the radial deviation of the conical tubular portion of the retainer from the pitch circle of the tapered rollers. In order to prevent separation of the tapered rollers from the inner race, the conical tubular portion of the retainer has to be radially outwardly deviated from the pitch circle of the tapered rollers. Thus, in order for the inner diameter D of the radially inner end of the protrusion 83a to be larger than the outer diameter D1 of the small-diameter annular portion 83b, it is necessary to reduce the radial dimension of the protrusion 83a and unnecessarily increase the radial dimension of the large end of the inner race that is brought into engagement with the protrusion 83a. Thus, the design of the bearing is restricted.